While I believe you are probably just loosing some emmersed leaves, (or have a nitrogen problem) here's a good diagram for figuring out plant deficiencies.

Macronutrients

Calcium (Ca)

* Symptoms: New leaves are distorted or hook shaped. The growing tip may die. Contributes to blossom end rot in tomatoes, tip burn of cabbage and brown/black heart of escarole & celery.
* Sources: Any compound containing the word 'calcium'. Also gypsum.
* Notes: Not often a deficiency problem and too much will inhibit other nutrients.

Nitrogen (N)

* Symptoms: Older leaves, generally at the bottom of the plant, will yellow. Remaining foliage is often light green. Stems may also yellow and may become spindly. Growth slows.
* Sources: Any compound containing the words: 'nitrate', 'ammonium' or 'urea'. Also manure.
* Notes: Many forms of nitrogen are water soluble and wash away.

Magnesium (Mg)

* Symptoms: Slow growth and leaves turn pale yellow, sometimes just on the outer edges. New growth may be yellow with dark spots.
* Sources: Compounds containing the word 'magnesium', such as Epson Salts.

Phosphorus (P)

* Symptoms: Small leaves that may take on a reddish-purple tint. Leaf tips can look burnt and older leaves become almost black. Reduced flowers or seed production.
* Sources: Compounds containing the words 'phosphate' or 'bone'. Also greensand.
* Notes: Very dependent on pH range.